The present invention is directed to coating materials which can be used to produce layers which are not only extremely oil- and water-repellent but also IR-reflective. The coating materials are particularly suitable as paints, e.g., as interior paints or masonry paints.
Easy-clean, dirt-repellent or self-cleaning layers are known in a number of variants. For example, there are purely hydrophobic coating systems which are based on nonpolar coating materials. Where not only an oil/fat-repellent but also a water-repellent finish is required, fluorine components are generally employed. Thus, DE-A-41 18 184 reported that layers containing fluorine or fluorosilane may exhibit anti-adhesion (hydrophobic/oleophobic) effects. Additionally, U.S. Pat. No. 5,760,126 describes fluorine-containing coatings, which are water-based.
One self-cleaning effect of surfaces has become known as the xe2x80x9clotus effectxe2x80x9d. It is based on the presence of certain, defined surface structures of hydrophobic materials and is modeled on nature, which brings about the dripping of water from the leaves of plants in a similar way (see EP 772 514 B1). Moreover, it has been reported that a combination of oleophobic and hydrophobic properties can be achieved by means of fractal structures which are disposed in the form of fluorosilane-treated Eloxal layers on aluminum.
The theory of wetting predicts that, at liquid contact angles on the substrate surface of more than 90xc2x0, roughening of the surface leads to reduced wetting. Below 90xc2x0, increased wetting occurs. For applications on transparent substrates such as glass, for example, roughnesses in the nm range are needed. For example, B. S. Hong et al., in Proc. 2nd Int. Conf. on Coatings on Glass (ICCG), Saarbrxc3xccken, 1998, pages 388 to 392 investigated a controlled separation of sol-gel systems for the purpose of generating nanoroughnesses and made use of its effect on wetting properties. In the Werkstoffwoche 99 Euromat session xe2x80x9cProperties and Technological Applications of Nanostructured Materialsxe2x80x9d, S. Sepeur et al. reported on the effect of the surface roughness of perfluorinated nanocomposite materials with high scratch resistance. The incorporation of silicon dioxide particles with a diameter of 230 nm significantly increased the hydrophobic and oleophobic properties of the surface. Instead of producing surface roughnesses by influencing the properties of the material, surfaces may also be structured by techniques such as embossing or the like. For instance, D. Sporn et al., in Specktrum der Wissenschaft, page 20-22, 1997, reported on ultrafine structures for antireflection coating which were xe2x80x9ctaken as seen from the eye of the mothxe2x80x9d.
Coating materials with low emissivity in the region of thermal radiation have been disclosed by G. Hugo in DE 44 18 214 C2. In that document, a binder with high transparency in the range from 3 to 50 xcexcm has incorporated into it a pigment which likewise possesses a high transparency within this range, but where the refractive index of the binder is different than that of the particles in the stated region of thermal radiation. Pigments proposed particularly include the fluorides, chlorides, selenides and sulfides of metals. The IR reflection maximum can be adjusted by way of the particle size and by the combination of pigments (DE 195 01 114 A1 and 196 50 300 A1). Pigment from brown rutile has also been described as exhibiting a high degree of infrared reflectiveness. DE 198 56 171 discloses the use of functional pearlescent pigments in transparent media which are suitable as coatings for the outside of buildings and which have angle-selective transmission and/or reflection properties in the visible region. The effect of IR-reflective pigments on the heat balance of buildings and in connection with military applications has been described by L. V. Wake in J. Oil Color Chem. Assoc., pages 78 to 81 (1990). Heat reflection may also be achieved through the use of hollow ceramic beads in a polymer matrix. Products having such ingredients are already being marketed for interior coatings; they are said to save up to 10% of heating costs.
DE 196 39 783 A1 to Merck, Hxc3xcls AG, describes metal oxide-coated pigments especially for water-based coating systems. The pigments have the structure of platelets atop whose topmost metal oxide layer there is an outer layer comprising certain oxides and/or mixed oxides. As the aqueous coating system, oligomeric silane systems are described. These systems develop silicon-functional hydroxyl groups which form chemical bonds with the hydroxyl groups of the pigment surfaces. The pigments do not have reflection properties in the infrared region.
U.S. Pat. No. 4,311,623 proposes the incorporation of zinc sulfide and other pigments into silicon-alkyd resins.
DE 197 26 862 A1 describes a process for producing a transparent protective layer, having reflecting properties in the infrared region, which can be used to coat transparent polymer plates or glass plates used in refrigeration or freezing apparatus. The protective layer is preferably of two-part construction, in which a thin metal sulfide layer is applied to the polymer or glass and overcoated with a so-called ORMOCER. An alternative proposal is to incorporate an additive having reflecting properties directly into such an ORMOCER.
It is an object of the present invention to provide coating materials from which coatings having excellent oleophobicity/hydrophobicity and, at the same time, heat-reflective properties can be produced. The materials should be suitable for processing by wet coating techniques and should preferably be self-curing, in order that they may be used as paints (e.g masonry or interior paints). Desirably, furthermore, they are solvent-free or contain only a small fraction of solvent. Where solvents are needed, they should as far as possible be nontoxic and environmentally friendly/readily biodegradable.
It has surprisingly been found that this object can be achieved through the provision of coating materials which comprise IR-reflective pigments having dimensions in the range from about 1 to 50 xcexcm or hollow beads having the same dimensions.
Before they are applied to the intended substrate, the coating materials should possess a viscosity at which they may be applied with conventional means such as rollers or brushes, by spraying, or otherwise. They must be or should be relatively mobile, such that following application the pigment particles in the near-surface layers are covered by a binder layer which is extremely thin, forms a stochastic surface structure, and yet is sufficiently thick to retain weathering stability. Viscosities like that possessed, for example, by honey when stored in the cold (i.e., which are sticky as honey), on the other hand, should be avoided. The coating material may be adjusted to an adequate viscosity where appropriate by separating off or adding solvent and/or dispersion medium (preferably water). In this way it is possible to dispense with additional surface structuring operations such as embossing or the like.
The dirt repellency properties (oleophobicity and hydrophobicity) of the layers formed from the coating material are improved still further by the binder of the coating materials and, respectively, of the coatings comprising at least one optionally organically polymerizable/polymerized silane of the formula (I)
XaRbSiR14xe2x88x92axe2x88x92bxe2x80x83xe2x80x83(I)
in which
X is a hydrolyzable group,
R is optionally substituted alkyl, alkenyl, aryl, alkylaryl or arylalkyl,
R1 is an organic radical which is attached to the silicon via carbon and carries a reactive group, especially one able to enter into organic reactions,
a is an integer from 1 to 3, and
b is an integer from 0 to 2,
or a (partial) condensate of this silane. Such (partially) condensed, optionally organically modifiable/modified silanes have been described in large numbers; the condensates, alone or in a mixture with further (partly) hydrolyzed metal compounds and/or organic components, are frequently referred to collectively as ORMOCERS(copyright). Particularly preferred such coating materials are those which are self-drying. Suitable self-drying materials are those, for example, formed from sol-gel systems. The silanes are hydrolyzed by adding an appropriate amount of water and are partly or fully condensed. Following the application of the material to a substrate that is to be coated, the solvent present evaporates or vaporizes. In this context it is also possible where appropriate to effect ultimate crosslinking and curing by means of further condensation. In some embodiments of the invention, the silanes have been or are additionally crosslinked with one another and/or with purely organic components of the coating material by way of organic groups, specifically by polyaddition or addition polymerization reactions. In order to circumvent the need to use heat or UV radiation to cure such systems, it is preferable in such cases to bring about organic crosslinking even before the coating material has been applied to the substrate. In these preferred cases it must of course be ensured that the viscosity of the material does not rise too sharply and/or that an appropriate amount of diluent or dispersion medium is added.
Highly suitable for the purposes of the present invention are optionally partially condensed silanes of the formula (I) having the following indices:
X is hydrogen, halogen, hydroxyl, an optionally substituted alkoxy, acyloxy, alkylcarbonyl or alkoxycarbonyl group having preferably 1-8, more preferably 1-4 carbon atoms in the alkyl or acyl group, NR3 where R3 is hydrogen, optionally substituted alkyl having preferably 1-8 and more preferably 1-4 carbon atoms or optionally substituted aryl; and R is an optionally substituted alkyl, alkenyl, aryl, alkylaryl or arylalkyl group having 1-10, frequently having 1-4 alkyl carbon atoms. In one particularly preferred embodiment, the radical R is partially substituted by fluorinexe2x80x94for example, in one of the abovementioned groups having from 6 to 10 carbon atoms and 4-8 fluorine atoms. Where a group R1 is present in the molecule, R1 preferably carries at least one amino, epoxide, vinyl or (meth)acrylate group. The presence of such groups frequently has the advantage that the pigments are incorporated more effectively in the matrix of the coating. For example, diols, which may come about by hydrolysis from epoxy groups, or amines have a high degree of affinity with the pigments that may be used.
Particularly suitable for the purposes of the present invention are coating materials comprising at least two different, optionally partially condensed silanes, in which case at least one silane of the formula (III)
XaRbSiR14xe2x88x92axe2x88x92bxe2x80x83xe2x80x83(III)
is used, in which X possesses the same meaning as indicated for formula (I), R1 possesses the same meaning as indicated for formula (I) and is preferably a C1-C6 alkyleneamine, and a+b is 3, and as crosslinking agent there is at least one silane of the formula (IV)
xe2x80x83XaSiRbxe2x80x83xe2x80x83(IV),
in which a is an integer from 1 to 4, a+b is 4, X is preferably a C1-C4 alkoxy and R is preferably a C1-C4 alkyl, or an oligomeric condensate of these silanes, which may have been formed by separate or joint hydrolysis of the silanes. An example of silanes of the formula (III) are omega-aminoalkyl-alkyl-dialkoxysilanes having C1-C3 alkyl groups; specific representatives of these silanes that may be mentioned include 3-aminopropyl-methyl-diethoxysilane or 3-aminopropyl-triethoxysilane. Examples of silanes of the formula (IV) are di-, tri- or tetraalkoxysilanes having C1-C3 alkoxy groups; specific representatives that may be mentioned include tetraethoxysilane and dimethyl dimethoxysilane. Examples of oligomeric condensates are siloxanes of the formula Rxe2x80x94[OSi(OR)2]nxe2x80x94OR where R is methyl or ethyl and n is 2 or 3, e.g., Dynasil 40 or Dynasil 50 from Hxc3xcls.
The silane(s) of the formula (I) and/or (III) or its/their (partial) condensates may suitably be present in an amount of about 5-95 mol %, based on all the constituents of the binder (e.g., all the constituents of the coating material with the exception of water, any further solvent and pigment). The amount is preferably about 10-50 mol %. Silane of the formula (IV) may, where present, be in an amount of preferably up to 50 mol %, based arithmetically on the total molar amounts of monomeric silane. The amount is preferably about 10-50 mol %, more preferably about 25-45 mol %.
Also particularly suitable for the purposes of the present invention are coating materials comprising one or more silanes of the above-indicated formula (IV) but without the addition of silanes of the formula (III), in which case these silanes form the xe2x80x9cframeworkxe2x80x9d of the condensed coating material. Specific examples of such silanes are dialkyldialkoxysilanes, e.g., dimethyldimethoxysilane or diethyldiethoxysilane, or alkyltrialkoxysilane, e.g., methyltrimethoxysilane or ethyltriethoxysilane, which may be used alone or in a mixture.
In one embodiment of the invention, the coating material comprises not only one or more optionally partially condensed silanes of the formula (I) (and/or (III) and/or (IV) as elucidated above) but also at least one metal compound of the formula (II)
M(OR2)nxe2x80x83xe2x80x83(II).
The metal M in this compound should be selected from those elements of the main groups and transition groups of the periodic system which are formally able to exist in the oxidation state 3 or 4. Preferably it denotes Ge, Sn, Pb, Ti, B or Al. Depending on valence, M [sic] denotes 3 or 4. R2 in this formula preferably has the same meaning as R in the formula (I). Since many metal compounds with the formal oxidation state 3 or 4 may also be present in the form of complexes with a multiplicity of ligands, the coating material may instead or additionally, however, include compounds as well in which some or all of groups OR2 of the formula (II) have been replaced by one or more ligands L which are bonded more strongly to the metal M than is the group (OR2). These compounds possess the formula (V)
(R2O)mMLnxe2x80x83xe2x80x83(V)
in which
M is with very particular preference Ti, Zr and Al, and/or
R2 is with very particular preference C1-C6-alkyl and/or
L is with very particular preference selected from 
m being an integer from 1 to 3 and
n being an integer from 1 to 3.
Preferably, compounds of the formula (II) and/or (V) are present in an amount of up to 50 mol %, based arithmetically on the total molar amounts of monomeric silane, preferably of 5-20 mol %.
In a further embodiment of the invention, in which the above metal compounds of the formula (II) and/or (V) may be present or not, and in which the silane or silanes may have any one of the aforementioned definitions, but are preferably present in the embodiments described as particularly suitable, the coating material further comprises at least one polymeric additive serving as a thickener or dispersant. Examples of such are polyvinyl alcohol, polyvinyl acetate, polyvinylbutyral copolymers of these substances in any desired combination, poly(meth)acrylates and copolymers thereof, (meth)acrylic/styrene copolymers, polyacrylic acid, acrylic acid-maleic acid copolymers, styrene/allyl alcohol copolymers, poylvinylpyrrolidone [sic] and vinylpyrrolidone/acrylic acid copolymers. These substances are preferably in the form of low polymers.
The abovementioned polymers may be present in the form of straight additives or, following modification and/or copolymerization with monomeric alkoxy silanes, may be connected to the silane condensate matrix as a consequence of hydrolysis and condensation. They are added advantageously in an amount of up to 80% by weight, based on the total amount of the constituents of the coating material with the exception of water and any solvent. An amount of 10-60% by weight is preferable.
It is also possible for organic monomers such as (meth)acrylates to be copolymerized with silanes of the formula (I) in which a+b is not more than 3 and the group R1 carries an organic polymerizable radical, e.g., a vinyl or methacryloxyalkyl group. Such silanes may, for example, be vinyldi- or -trialkoxy silane or methacryloxypropyltrimethoxysilane. Their amount may be situated in the region of that which was stated above for the organic polymers.
Instead of this or additionally it is possible for organic polymers like those mentioned above to be modified subsequently with appropriate silanes. One example of this is the modification of polyvinyl alcohol with 3-triethoxysilylpropylsuccinic anhydride or with isocyanatopropyltriethoxysilane; 
This example may also be applied to other mentioned substances, e.g., to polyvinylbutyral.
A further possibility of polymer modification is the reaction of polymers containing acid groups, such as polyacrylic acid, with amino silanes, e.g., H2Nxe2x80x94(CH2)xxe2x80x94Si(OR)3 where x is preferably 1-4 and R is preferably methyl or ethyl, to give the acid amide with hydrolyzable groups.
A further improvement in the dirt repellency effect is achieved through the incorporation of fluorinated alkyl chains which, as mentioned above, may be bonded to the silanes of the formula (I), but may also be incorporated into the coating material through the addition of fluorinated organic compounds. Indeed, it is not critical whether the fluorinated groups are attached to the silicon network and/or to the organic polymer networkxe2x80x94which may be linked to the silicon network. It is preferred for the fluorine groups to orient themselves toward the surface of the coating produced from the coating material. In one preferred embodiment, the coating material comprises optionally partially or fully hydrolyzed/condensed silanes of the formula (IV) as described above, together with silanes of the formula (I) in which R is a partially fluorinated alkyl radical, preferably a C6-C10 alkyl radical having about 4-16 fluorine atoms, preferably with 4-8 CF2 groups, and also, if desired, one or more metal compounds of the formula (II) and/or (V).
A particularly good beading effect is achieved through the incorporation of hollow melamine/formaldehyde or urea/formaldehyde microbeads having diameters of, e.g., 1-30 xcexcm into the coating material of the invention. With coating materials which include such hollow beads it is possible to measure wetting angles of up to 145xc2x0.
The IR-reflecting pigments which may be used in the present invention are not subject to any restriction, provided they are present substantially or to large extents (preferably more than 50%, more favourably more than 75%) in the sizes specified above. Highly suitable for the invention are electrically conducting metal spangles, which may be coated for the purpose of imparting color and/or preventing corrosion, or dielectrics having a sufficiently large difference in refractive index (preferably at least 0.1, more preferably about 0.2 to 0.3) from the binder. It is also possible to use hollow, open or closed dielectric beads whose refractive index is in this case close to 1 and which develop their maximum activity as centers of scattering in binders having a relatively high refractive index. It is particularly preferred to use pigments having high reflectivity in the wavelength range between 8 and 14 xcexcm. Where coating materials having these properties are in fact used as masonry paint, there is a particular reduction in the radiation losses, as a result of which the surface temperature rises and the heat losses of, say, the painted house fall. Moreover, there is a reduction in the frequency with which the temperature falls below the dew point. This also is advantageous, since dew deposition leads to water absorption and soiling.
Examples of suitable infrared-reflecting pigments that may be mentioned include metals and metal alloys, especially aluminum, chromium, iron, copper, nickel, silver, gold, iron, tin, zinc, bronze, brass, ferrous alloys, and nickel alloys. Metal sulfides as well may be used, especially copper, lead, and zinc sulfide, and also metal fluorides, especially calcium fluoride, barium fluoride and sodium fluoride, and metal oxides, especially zinc oxide, titanium dioxide, magnesium oxide or antimony dioxide. Examples of dielectric pigments are ceramic or vitreous pigments.
The amount of pigment in the coating material is advantageously at least 5% by weight, based on all the constituents of the coating material with the exception of water and any further solvent; it is advantageous to choose up to 60% by weight, and with very particular preference 15-50% by weight.
As already mentioned, the pigments themselves may have been appropriately coated or surface-derivatized. If the surface exhibits hydroxyl groups, the pigments may for example be incorporated into the network, thereby removing the possibility of them possibly settling on prolonged standing. Moreover, they may have been provided with a colored layer, if they do not themselves have an appropriate color. By this means it is possible to produce any desired color effects, for masonry paints, for example. Naturally, a masonry paint may also be whitexe2x80x94as a result, for example, of the addition of gypsum, talc, chalk or titanium dioxide.
Infrared-selective masonry paints influence the radiation balance of the outsides of buildings and therefore have consequences for the surface temperature of the facade. As a result, it is possible to achieve important effects, some of which have already been elucidated above, for the utility of the building. Surprisingly, however, in the course of the use of the coating materials of the invention, an effect which is above all synergistic has been established. As a result of the dirt-repellent surface of the interior and exterior coatings produced from or with the coating materials of the invention, indeed, the infrared reflection effect is retained for longer, since instances of soiling generally have a higher degree of thermal emission and hence increase the heat radiation given off. Through the combination of IR radiation and fat/oil and water repellency properties, the periods of dewing are reduced and the wetting or soaking of the outer skin of the building is significantly lowered. Conversely, the pigments also bring about a synergistic improvement in the dirt repellency effect of the masonry paint, since pigments comprising a hydrophobic/oleophobic matrix have a very good beading effect with respect to water droplets (contact angles in the region of approximately 130xc2x0 may be measured on coatings even without hollow organic microbeads).
The use of organically modified silanes as starting compounds in the coating materials makes it possible in many cases, moreover, to operate in exclusively or substantially aqueous systems, thereby making the materials of the invention environmentally compatible. The coatings obtained have a high weathering stability.
To prepare the coating materials of the invention, it is possible to start by introducing the chosen silanes, in monomeric form or already in partially condensed or organically prepolymerized form, in a solvent, which is preferably water. It may be necessary to add water to the solvent. Following hydrolysis and condensation, organic monomers or polymers may be added if appropriate. Where they are to be polymerized onto the silane(s) by way of organically polymerizable groups, the corresponding reaction is made possible, by for example adding initiators and exposing the system to heat or UV radiation. Where metal alkoxides of the formulae (II) or (V), possibly in complexed form, are to be added, it is advantageous to effect this as the next step, by for example adding a corresponding alkoxide in an optionally complexing solvent such as acetone or acetylacetonate. Conversely, it is of course also possible to add the silane or silanes in monomeric or precondensed form to a mixture of optionally complex metal alkoxide of the formula (II) and (V) and, if desired, organic monomers or polymers in an appropriate solvent or complexing agent and then to add the amount of water necessary for the hydrolysis. Finally, the intended amount of pigment is added. Where necessary, the viscosity of the resulting mixture is raised or lowered by adding or removing water or other solvent. Furthermore, provision may be made to replace any water present by another, preferably low-boiling solvent, in order to facilitate or accelerate the process of drying of the applied coating.
With this sol-gel preparation process, a coating material is obtained which may be used, for example, as an interior or masonry paint or may be admixed to conventional paints of this kind. Following application to a substrate, by rolling, brushing or knife coating, for example, the material may be dried and, where appropriate, cured until it is no longer tacky.